When I use MathJax, sometimes the fraction lines got a different thickness (below imgage). Why?

Another question is how to increase the vertical space between the lines, because in \begin{align} ... \end{align} the lines are so close to each other and it's hard to read them (as you can see on the below sample image).

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MathJaX doesn't use the same program as TeX so questions about MathJaX aren't really on topic here. You will probably have more luck at the MathJaX Help Forums
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Loop SpaceOct 4 '12 at 18:59

See my response here for details of why this happens in MathJax.
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Davide CervoneOct 5 '12 at 15:43

3

Ps, you can use \\[dimen] to add extra space between lines of an alignment. So \\[5pt] would add an extra 5 points of space between the lines at that breakpoint.
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Davide CervoneOct 10 '12 at 17:15